Rollup merge of #132473 - ZhekaS:core_fmt_radix_no_panic, r=joboet

[core/fmt] Replace checked slice indexing by unchecked to support panic-free code

Fixes #126425

Replace the potentially panicking `[]` indexing with `get_unchecked()` to prevent linking with panic-related code.

library/core/src/fmt/num.rs

@@ -88,7 +88,10 @@ unsafe trait GenericRadix: Sized {
                 };
             }
         }
-        let buf = &buf[curr..];
+        // SAFETY: `curr` is initialized to `buf.len()` and is only decremented, so it can't overflow. It is
+        // decremented exactly once for each digit. Since u128 is the widest fixed width integer format supported,
+        // the maximum number of digits (bits) is 128 for base-2, so `curr` won't underflow as well.
+        let buf = unsafe { buf.get_unchecked(curr..) };
         // SAFETY: The only chars in `buf` are created by `Self::digit` which are assumed to be
         // valid UTF-8
         let buf = unsafe {